The present invention relates to semiconductor device fabrication and integrated circuits and, more specifically, to methods of self-aligned double patterning and interconnect structures formed by self-aligned double patterning.
A back-end-of-line (BEOL) interconnect structure may be used to connect device structures, which have been fabricated on a substrate during front-end-of-line (FEOL) processing, with each other and with the environment external to the chip. Self-aligned patterning processes used to form a BEOL interconnect structure involve linear mandrels acting as sacrificial features that establish a feature pitch. Non-mandrel lines are arranged as linear spaces between sidewall spacers that are formed adjacent to the sidewalls of the mandrels. After the mandrels are pulled to define mandrel lines, the sidewall spacers are used as an etch mask to etch a pattern predicated on the mandrel lines and the non-mandrel lines into an underlying hardmask. The pattern is subsequently transferred from the hardmask to an interlayer dielectric layer to define trenches in which the wires of the BEOL interconnect structure are formed.
Mandrel cuts may be formed in the mandrels in order to section the mandrels and define discontinuities between the sections. Non-mandrel cuts may also be formed along non-mandrel lines and may include portions of the spacer material used to form the sidewall spacers. The mandrel cuts and non-mandrel cuts are included in the pattern that is transferred to the hardmask and subsequently transferred from the hardmask to form the trenches in the interlayer dielectric layer. The mandrel cuts and non-mandrel cuts appear in the BEOL interconnect structure as adjacent wires that are spaced apart at their tips with a tip-to-tip spacing related to the dimension of the discontinuity.
The tip-to-tip spacing for a cut mandrel is limited to a distance equal to twice the thickness of the sidewall spacers. If the tip-to-tip spacing is greater than this distance, the sidewall spacers do not merge within the mandrel cut between the sections of the mandrel, which results in incomplete filling of the mandrel cut. Transverse to the length of the cut mandrel, the mandrel cut is arranged in the pattern between non-mandrel lines that flank the cut mandrel line. The result of the incomplete filling can be a short between wires in the BEOL interconnect structure formed using the non-mandrel lines at the location of the mandrel cut. In addition, the wrapping of the sidewall spacers about the tips of the sections of the cut mandrel may introduce notches or indents at the side edges of the merged sidewall spacers. These notches or indents may form metal asperities that project from the side edges of wires formed using the adjacent non-mandrel lines into the mandrel cut. The proximity of these metal asperities may also result in shorting of wires in the BEOL interconnect structure formed using the non-mandrel lines.
Improved methods of self-aligned double patterning and improved interconnect structures formed by self-aligned double patterning are thus needed.